The present invention relates to a polyester laminated film. More particularly the invention relates to a polyester laminated film which enables easy discrimination between the films by visual and/or optical means owing to incorporation of fine closed cells in the film surface layer and also has high-degree image printability in thermosensitive transfer recording. The present laminated film, therefore, finds particularly useful application as a base for various kinds of cards, bills and the like.
Polyester films, especially polyethylene terephthalate films, have been widely used as base for various kinds of commercial products, for example, as base for information recording media, condensers, printing plates for planography, packages, etc., owing to their excellent mechanical, electrical and surface properties, high chemical resistance and relatively low cost. For instance, white high-opacifying biaxially stretched polyester films are used because of their excellent properties, as base for magnetic cards such as telephone cards and orange cards (traffic fee prepaid cards) and for thermosensitive transfer recording media in the field of hard copies.
With a great spread of telephone cards or orange cards, introduction of prepaid cards has been considered positively in various types of business, such as service business, restaurant business, marketing business, etc., in recent years. Expansion of the scope of use of the cards and the increase of their users have brought about the problem of visual discrimination of one type of card from the other and, more seriously, the problem of security of the cards, that is, judgement of genuineness of the individual cards and prevention of forgery. Improvement on these matters has now become an important social requirement. Various attempts have been made for improving security of cards, such as storage of personal identity numbers in a coated magnetic strip and combination thereof with bar code or card punch. These methods, however, are not always satisfactory, and researches for a method with a higher degree of reliability are made.
Under these circumstances, it is required that the polyester film used as card base be provided with a forgery preventing ability or a function enabling easy judgement of genuineness.
Hitherto, various types of images such as letters, characters, figures have been printed on the card surface for allowing visual discrimination between the different types of cards. However, the method of directly printing an image on the card surface by thermosensitive transfer printing has not yet been applied to practical use. This is for the reason that because of rigidity of the base polyester film surface, transfer performance is bad and there may occur unevenness of image printing or partial omission of the printed image, making this method impractical. Improvement of this technique, therefore, has been strongly required.
As a base film that can give a solution to said problems, the present inventors have proposed a highly opacifying white polyester film containing a large number of fine closed cells all over the base film.
However, this film, although capable of improving transfer performance, is still not always satisfactory in respect of security. Thus, the development of a base film that can meet both requirements has been desired.
In view of these circumstances, the present inventors have pursued further studies for providing a base film which is excellent in both security characteristics and print transfer performance and, as a result, have accomplished the present invention.